In the operation of a pulverized coal-fired boiler, a significant fraction of the ash contained in the coal is deposited on the water walls of the combustion chamber and on the heat exchange tubes of the convection section of the boiler. The ash deposits have a low thermal conductivity, modify the radiative properties of the surfaces and insulate the tubes from the flame and from the combustion gases. These effects interfere with the efficient gas-to-tube heat transfer to both the furnace walls and the convection section tubes.
In U.S. Pat. No. 4,408,568, in which two of us are named as inventors and which is assigned to the assignee hereof, there is described a method of monitoring the build up of ash on the inside walls of a coal-fired boiler by simultaneously determining the actual heat flux present in the boiler and the heat flux reaching the walls of the boiler, and determining the difference in heat flux value as a measure of the build up of ash on the inside walls. The signal indicative of the degree of furnace fouling may be used by a furnace operator as a determination for initiation of soot blower operation and/or other furnace control action, or may be utilized for automatic initiation of soot blower operation or other boiler control.
In the convection section of the steam generator, heat is removed from the combustion gas stream by convection and conduction through the walls of tubes contacted by the gas stream and through which steam flows. Usually banks of heat transfer tubes are provided which are serially contacted by the flowing gas stream. The function of the convection section usually includes superheating pressurized steam prior to passage to a turbine driven by the steam to produce power, and re-heating of low-pressure steam returned from the high-pressure side of the turbine, prior to recycle to the low-pressure side of the turbine.
As noted above, ash deposition also can occur on the tubes in the convection section of the boiler. At present, no direct means is being provided for assessing the amount of ash being deposited in the convection section and the degree to which the deposit has decreased the ability of the heat exchange surfaces to transfer the heat from the gas phase to the steam.
Ash deposition, moreover, may occur unevenly. Across one particular horizontal plane of a tube bank, there may occur more fouling in one side or corner than in another, causing an uneven distribution of gas flow, usually called channelling. In the present manner of operating steam generators, there is provided no means to identify the degree of unevenness of the fouling.
An operator relies on a number of indirect signals and the occasional visual inspection to determine when to operate soot blowers to remove accumulations of deposited ash from the tubes in the convection section. The lack of more direct information has led to inefficiencies, upsets in control leading to non-steady operation, and occasionally catastrophic fouling necessitating shutdown. In addition, there is considerable needless or excessive soot blowing of convection section tubes which are actually clean. Soot blowing erodes the heat-exchange tubes, so that much needless soot blowing is detrimental and costly.
There are diagnostic systems being marketed which are based on measuring the conditions at the exit of the boiler. These systems permit only an indirect measure of fouling and, since response times are long, the signals are generally inadequate to achieve satisfactory control.
There is a need, therefore, to provide a direct means of measuring ash build up in the convection section of steam generators, so that boiler operation can be improved.